The present invention relates generally to audio signal processing components, and more specifically to a "direct box" for coupling instrument audio signals having unbalanced high-impedance formats to audio console inputs designed to receive balanced low-impedance audio signal formats.
Popular musical instruments in common use today, such as electric guitars and acoustic instruments, typically employ a magnetic pickup, a transducer, or a microphone through which sound produced by the instrument is converted into an electrical audio signal for amplification, processing, and recording.
In some musical arrangements, such as on-stage performances, the instrument being played is typically situated at a substantial distance away from the electronic equipment used for amplifying, processing, and recording the audio signals, necessitating a lengthy transmission line. With the exception of a few high-cost professional instruments, most musical devices employ pickups and microphones which are characterized as high-impedance, unbalanced devices, having impedance in the range of 3 k.OMEGA. to 50 k.OMEGA.. Compared against the low-impedance balanced microphones found in professional instruments, having impedance on the order of 0.300 k.OMEGA. or less, these high-impedance devices and the lengthy transmission lines connecting them are susceptible to external hum fields and signal noise which can substantially degrade the audio signal quality.
Typical environments contain multiple AC 60-cycle power sources which can produce hum fields. These fields can be introduced into the audio signals through single ended high impedance sources in the audio setup. The degree of hum not only adds to the overall signal noise present in the audio system, but reduces the dynamic range of the audio signals in the system by reducing the signal level difference between the lowest note and the quietest note.
Impedance matching transformers have long been used reduce external hum fields and signal noise over transmission lines by converting electrical audio signals from the high-impedance, unbalanced formats employed by common musical instruments into low-impedance, balanced formats for better audio signal quality, such as is shown in U.S. Pat. No. 5,612,646 to Berning. However, the use of impedance matching transformers has several disadvantages. The transformers necessitate components of a larger physical size and higher cost, as well as suffer from a reduction in dynamic signal ranges, an increase in signal distortion level, and the possibility of noise saturation due to transformer overload. Additionally, a typical transformer coupled direct box cannot respond to the leading edge transients contained in musical audio signals, creating an audible output signal distortion. For example, a musical instrument like the English horn is capable of reaching 60% of its normal peak amplitude in 5 ms. Other instruments can often have rise times that vary from 50 ms to 200 ms, and a hand-clap can rise as fast as 0.5 ms. Typical transformers are not capable of producing an adequate signal response within these short time periods
One alternative to an impedance matching transformer includes the use of "direct box" active electronic circuits employing solid-state transistor circuitry or vacuum-tube circuitry to couple the high-impedance instruments directly to the low-impedance audio components. Active electronic devices incorporating vacuum tubes, such as shown in U.S. Pat. No. 5,343,159 to Butler, suffer several disadvantages, including higher manufacturing costs, reliability problems, and an increase in audio signal distortion levels. Similarly, active electronic devices employing solid-state transistor circuitry are typically complex and costly designs, requiring an external AC power source such as a wall outlet, or a battery-type power supply.
Once such device, shown in U.S. Pat. No. 4,737,735 to Kampes for a "Phantom Powered Amplifier" utilizes a non-inverting amplifier and voltage follower circuit. This configuration is a unity-gain stage wherein the feedback from the inverted stage to the output stage is increased to infinity and the feedback is always 100%. The output of this circuit is a unity-gain phase replica of the input voltage, and can degrade the audio signal through excessive feedback. To compensate for this 100% feedback, the '735 patent discloses the use of an input pad floated above electrical ground to reduce input levels to the circuit, and requires additional circuit components.
Accordingly, there is a need in the music industry for a low-cost and reliable direct box audio signal converter which does not induce signal distortions through the use of an impedance matching transformers or vacuum tubes or require the use of an AC power source or battery. Furthermore, a circuit which would allow for adjustment of the feedback gain, either gain reduction or gain increase would greatly improve the quality of the audio signals passing through the circuit.